1. Field of the Invention
The present invention relates to a multi-stage optical amplifier and a method of controlling the same.
2. Description of the Related Art
Optical fiber communication is an indispensable technique for point-to-point (which connects computers to networks through telephone lines) transmission or high-speed and high-capacity communication in metro/access networks, and many techniques have been developed for the optical fiber communication.
In recent years, for example, WDM (wavelength division multiplexing) and optical amplifiers have been used to support the optical fiber communication. WDM is a technique that makes the spectrums of optical pulses to be orthogonal to each other on a frequency axis and transmits the optical pulses. The method can considerably increase transmission capacity and be expected to improve the utilization of frequencies. An erbium doped optical fiber amplifier that was put into practical use in 1990s makes it possible to compensate for the loss of an optical fiber periodically and contributes to a considerable increase in transmission distance. An optical amplifier achieved by combining the two techniques includes a function of making the gain in a frequency band constant (Japanese Patent Application Laid-open Nos. 2003-174421 and 2006-166478).
However, spontaneous emission noise generated from the optical amplifier causes problems in achieving high-speed and high-capacity communication. A noise factor is an index indicating the degree of the deterioration of signals due to the spontaneous emission noise.
In general, since the optical amplifier performs total power control, it detects all optical energy, which makes it difficult to discriminate signal light from spontaneous emission noise. As a result, even when the gain of the optical amplifier is controlled to a desired value, it is difficult to control the gain of signal light to a desired value. For this reason, it is necessary to increase the gain in consideration of spontaneous emission noise generated. A spontaneous emission noise correction amount is defined as a parameter for correcting the deterioration of signals due to the spontaneous emission noise. It is possible to make the gain of signal light constant by adding the spontaneous emission noise correction amount to the output power.
Here, the total power is optical output including noise, and the signal light power is optical output without noise.
However, in the multi-stage optical amplifier performing constant gain control in which the gain in each stage is made constant and an output unit corrects the spontaneous emission noise, when the number of wavelengths is small and input power is low, the gain of signal light in the first stage becomes excessively small, and a noise factor deteriorates (an SN ratio is lowered), thereby causing a problem.